Two roads were blocked by landslides, and waters and mud reached almost 2-metre high in the hardest hit districts of southern Sulawesi province, said rescue official Abdul Malik on Thursday.
In the worst-hit region of Sinjai, 175 people were killed, while 15 others perished in neighbouring regions, said local government spokesman Annas.
Ode Parmodes, an official at the island's disaster relief co-ordination office, said: "Search and rescue efforts are still under way to find missing bodies and evacuate people from devastated areas, but rescuers say most of the missing people are likely to have been swept out to sea."
The flash floods and landslides were triggered by rains since Monday, and the government has promised an investigation into claims that illegal logging may have been a contributory factor.
Malam Kaban, the forestry minister, said that "what has happened in Sinjai should become a lesson to all of Indonesia: People must be alert if torrential rains pour over areas where forests have been depleted."
Hundreds of people flocked to hospitals to look for missing relatives, witnesses said.
Sinjai district was the worst hit,
with 175 people killed
A survivor called Rohim recounted being swept out to sea after a flood tore through his house early on Tuesday morning.
He survived for nine hours by hanging on to a piece of driftwood, but his wife and two sons were still missing.
"I pray for them, and hope rescuers can find them," he said at Sinjai hospital, where he had been looking for their bodies in the mortuary.
"I will stay here until I can find them - dead or alive."
Seasonal downpours cause dozens of landslides and flash floods each year in Indonesia, where millions of people live in mountainous regions and near fertile flood plains close to rivers.
Some environmentalists and government officials blame widespread deforestation, which they contend loosens soils on mountainsides.
A ferry with 120 people on board has sunk off Sumatra island in western Indonesia because of bad weather, a senior rescue official said on Thursday, but there was no information on the status of the passengers.
Dianta Bangun, head of the search and rescue office in North Sumatra province, told Reuters that rescuers were still trying to find the boat, which he said had 108 passengers and 12 crewmen.
A navy official told a Jakarta-based radio that two Americans were on board the vessel.
"The information is it has sunk between Sumatra and Nias islands. The cause was a storm," said Bangun.